Dehydrofluorination of polyfluoroalkanes



Patented F b. 15, 1949 DEHYDROFLUORINATION OF POLYFLUOROALKANES DonaldDrake Coi'fman and Richard D. Cramer, Wilmington, Dcl., assignors to E.L du Pont de Neniours 8; Company, Wilmington, Del., a corporation ofDelaware No Drawing. Application December 4, 1946, Serial No. 713,924

- This invention relates to the catalytic dehydrofluorination ofpolyfluoroalkanes and, more particularly, to the dehydrofluorination of1,1-difluoroethane to vinyl fluoride.

Heretofore the dehydrofluorination of polyfluoroalkanes to fiuoroalkeneshas been accom-' plished by simple pyrolysis and by pyrolysis-in thepresence of various metals or salts as catalysts (application Ser. No.633,264, filed December 6, 1945, by Downing, Benning and Mel-larness;application Ser. No. 633,265, filed Decemher 6, 1945, by Harmon). Ingeneral, these methods require high temperatures and/or relatively longcontact times, with the result that undesirable' side reactions occurwith the formation of acetylenic compounds which contaminate thereaction product. -In particular, the presence of more than very smallamounts of acetylene in vinyl fluoride is undesirable and necessitateseither a careful fractionation of the two gases or an expensivescrubbing of the mixture with acetylene absorbents. Furthermore,pyrolysis of polyfiuoroalkanes, particularly at relatively long contacttimes, tends to involve formation of tarry deposits which usuallydeactivate the catalysts employed. Removal of the tarry or carbonaceousdeposite is sometimes impossible and in any event does not always resultin reactivation of the catalysts.

An object of the present invention is to provide an improved process ofdehydrofluorinating polyfluoroalkanes to fluoroalkenes. Anotherobject isto provide a catalytic dehydrofluorination process wherein the catalystpermits very short contact times, thus minimizing the formation ofacetylenic compounds, and wherein the catalyst can be reactivated byremoval of tarry or carbonaceous deposits. A still further object is toprovide such an improved process of dehydrofluorinating1,1-difluoroethane to vinyl fluoride. Other objects will be apparentfrom the description of the invention given hereinafter.

The above objects are accomplished according to the present invention byheating a polyfluoroallra'ne containing at least two carbon atoms withat least two fluorine atoms attached to one carbon atom and at least onehydrogen atom on an adjacent carbon atom at a temperature of Mil-850 C;in contact with chromium trifluorlde at a contact time of less thanthree minutes, and

- recovering the iiuoroalkene formed. Polyfluoroalkanes having from twoto four carbon atoms, "inclusive, and no substituents other thanfluorine, are particularly well adapted to the process of thisinvention. Preferably, a temperature of 500- 5 Claims. (Cl. 260-653) 750c. is employed with a contact time of less than two seconds.

A convenient method of practicing this invention isto pass thepolyfluoroalkane over massive chromium trifluoride contained in atubular reactor heated at 500-750 C. undersubstantially atmosphericpressure. The iluoroallrene is recovered by distillation and therebyseparated from the hydrogen fluoride formed and from unreactedpolyfluoroalkana' In general, there will not be enough acetyleniccompounds formed to necessitate their, removal, but this can'bedone ifdesired by scrubbing the product with a suitable absorbent such asammoniacal cuprous chloride. Alternatively, the pyrolyzate may be passedthrough an absorbent for hydrogen fluoride, such as soda-lime, then, ifdesired, through an absorbent for' acetylenic compounds, and thefluoroalkene separated from unreacted polyfluoroalkane by distillation.Suitable apparatus comprises means for continuously metering thepolyfiuoroalkane, such as a flow-meter or rotamete'r, a tubular reactorof hydrogen fluoride-resistant material, e. g., Inconel, nickel orplatinum, towers for scrubbing the pyrolyzate if desired, and afractionating column.

Polyfluoroalkanes of suitable quality for the process of the instantinvention may be prepared by a number of methods. For example, l;l-difluoroethane may be prepared by the process disclosed in applicationSerial No. 633,556, filed December 7, 1945, in the names of Burk,Coliman and Kalb, now U. S. Patent No. 2,425,991, said processcomprising reacting acetylene with hydrogen fluoride in the liquid phasein the presence of catalytic amounts of boron trifluoride undersubstantially anhydrous conditions. Other polyfiuoroalkanes may beprepared by replacing chlorine in polychloroalkanes by fluorinejby meansof such fluorinatingagents as hydrogen fluoride, antimony fluoride, andthe like.

The following examples, in which all parts are by weight unlessotherwise specified, illustrate specific embodiments of the presentinvention.

inside, diameter and having a heated zone '7 inches long was packed withchromium trifiuoride pellets prepared by pelleting hydrated chromiumtrifluoride (CIF3'3H2O) with 2% of polytetrafluoroethylene as a binderand firing the pellets at 850 C. in. an atmosphere of hydrofluoric acid.1,1-difiuoroethane was passed over this catalyst at 725 C. and at a rateof 16 cc. per

3 second. The linear velocity was about 35 cm. per second, correspondingto a contact time of 0.38 second. The crude pyrolyzate was passedthrough a tower fllled with soda-lime to remove hydrogen fluoride, thenscrubbed in a counter-current of silver nitrate solution to determineits acetylene content. The scrubbed gases were condensed in a trapcooled with solid carbon dioxide and acetone, and the condensate wasdistilled through a precision column. From 487 parts of1,1-difluoroethane there was obtained 111 parts of vinyl fluoride,corresponding to a conversion of 32.7%. There was recovered 310 parts of1,1-difluoroethane, indicating a 90% yield of vinyl fluoride based onthe 1,1-difluoroethane actually consumed. Analysis of the silver nitratesolution indicated that the product contained 2800 parts per million oracetylene.

when, instead of massive chromium trifluoride, there was used asupportedcatalyst prepared by impregnating'absorbent carbon with asuspension of chromium trifluoride in absolute ethanol and drying at 140C. for 8 hours under reduced pressure, there was obtained a 28%conversion at 725 C. and 0.44 second contact time. The product contained3600 parts per million of acetylene.

When 1,1-difiuoroethane was pyroiyzed in the same reactor, but withoutcatalyst, at 725 C. and 0.44 second contact time, the conversion tovinyl fluoride was only 3.3% and the product contained 10,000 parts permillion of acetylene.

It is advantageous in catalytic gas phase processes to use a high gasvelocity; whenever possible, since this results in a higher throughputof the material treated. The advantages of the chromium trifluoridecatalyst are realized also at high gas velocity, as shown by thefollowing example.

Example II A tubular Inconel reactor of inch inside diameter and havinga heated zone 35 inches long was packed with the pelleted anhydrouschromic fluoride catalyst of Example I. Thereactor was heated to 600 C.and 1,1-difluoroethane was passed overthe catalyst at the rate of 55 cc.per second with a linear velocity of approximately 175 cm./second.corresponding to a contact time of 0.61 second. The pyrolyzate was freedof hydrogen fluoride and acetylene in the manner described in Example I,condensed and distilled. From 576 parts of 1,1-difluoroethane wasobtained 95 parts of vinyl fluoride and 422 parts of unchanged material,corresponding to a conversion of 24% and a yield of 88.5%. Analysis ofthe silver nitrate solution indicated that the product contained 270parts per million oi acetylene. Under the same conditions but at atemperature of 725 0., conversion was much higher (38%) but there wasmore acetylene in the eiliuent gas.

In comparison, when 1,1-difluoroethane was pyrolyzed in the samereactor, but in the absence of chromic fluoride, at 680-690 (3., gasvelocity about 175 cmJsecond and 0.49 second contact time,'theconversion was only 9% and the product contained more than twice theamount of acetylene formed in the presence of the catalyst.

Example III was V 1. 4 packed with the pelleted anhydrous chromicfluoride catalyst of Example I. The crude pyrolyzate was passed throughsoda-lime to absorb hydrogen fluoride and then condensed in a trapcooled with a carbon dioxide/acetone mixture. In order to determineaccurately the amount of fluorobutene formed, the reaction mixture wasbrominated atia temperature of 30 C. and the resultingdibromofluorobutane was separated from the unreacted difluorobutane byrectification. There was obtained, in addition to parts of unreacted2,2-difluorobutane, 36 parts of dibromofluorobutane, corresponding to aconversion of difluorobutane to monofluorobutene of 12.7% and a yield of6.0%. When the same reactor, but without catalyst; was used at the sametemperature and contact time, the conversion to fluorobutene was only4%.

The ready and convenient reactivation of the chromium trifluoridecatalyst after its activity has been impaired by deposition thereon oftarry or carbonaceous materials is illustrated bythe following example.

Example IV Using the reactor and catalyst of Example II and the samehigh gas velocity (about cm./second) a series of three runs was madestarting with fresh -catalyst which was not changed or cleaned betweenthe runs. In these three runs, 1,1-difluoroethane was passed over thecatalyst at temperatures of 500, 550 and 550 C., respectively, atcontact times of 0.62, 0.64, and 0.66 second; respectively. Theconversions to vinyl fluoride in the three consecutive runs were 14%, 7and 5%, respectively, and the acetylene contents of the product were250, 480, and 500 parts per million, respectively, showing that thecatalyst was increasingly deteriorating as its contamination with tarrydeposits increased. At the end of the third run, the cataylst was heatedat 550 C. in a stream of oxygen which was passed through the reactor fortwo hours. 1,1-difiuoroethane was then passed through at the same gasvelocity, at a temperature of 550 C. and a contact time of 0.56 second.The conversion to vinyl fluoride was 17% and the gas contained only 300parts per million of acetylene.

It will be understood that the examples set forth herelnbefore aremerely illustrative and that the present invention broadly comprisesheating a polyfluoroalkane containing at least two carbon atoms with atleast two fluorine atoms attached to one carbon atom and at least onehydrogen atom on an adjacent carbon atom at a temperature of 400-850" C.in the presence of chromium trifluoride. Examples of other suitablepolyfluoroallranes are 1,1,1-trifluoroethane 2,2-difiuoropropane, and1,1-difluorobutane. Polyfluoroalkanes containing two to four carbonatoms inclusive, and having no substituents other than fluorine havebeen found to be especially well adapted to this invention.

- While massive chromium tr'ifluoride is the preferred catalyst,chromium trifluoride supported on activated carbon or on other materialsinert to hydrogen fluoride is also highly eflicient. Instead of chromiumfluoride there may be used, although somewhat less efllciently, chromicoxide or hydroxide, which are believed to be converted at least in partto chromium fluoride in the reaction zone under the influence of thehydrogen fluoride formed on pyrolysis of the polyfluoroalkane. Chromiummetal also may be used as a catalyst but it is not as desirable aschromium trifluoride from the standpoint of conversion I and acetyleneformation.

Although the catalytic dehydrofluorination of polyfluoralkanes can beaccomplished according to this invention at temperatures between 400 C.and 850 C., a reaction temperature between about 500 C. and about 750 C.is preferred. Below 500 C., the conversion to fiuoroalkenes' is apt tobe low. while above 750* C. side reactions, especially those conduciveto acetylene formation, tend to be excessive. J

As has been shown, the chromium trifluoride catalysts permit the use ofvery lowcontact times. Low contact times are important not only becausethe purity of the product is thereby increased but also for economicreasons. The preferred contact .me (which, of course, depends somewhaton the size of the reactor) is between- 0.25 and 2 seconds, morepreferably still between 0.25 and 1 second. However, much shorter ormagi longer contact times, varying between 0.02 andI80 seconds, may beused if desired, depending on other reactionconditions and on thematerial being treated.

All contact times given herein are calculated with the gas volume takenat standard temperature and pressure conditions, i. e., C. and 760 mm.,and with the volume of the reactor corrected for the volume oi catalystcontained therein.

The pressure conditions oi the reaction mayfluoroalkane at atemperatureof 400 in the presence of chromium trifluoride.

limits. In general, any gas velocity above about 0.05 meter/second issuitable, provided the contact timelimits which have been set forthhereinbeiore are not exceeded. It is advantageousto use as high a gasvelocity as possible. Good results are obtained with. gas velocitieswhich,

. while high, are still in the viscous flow region, i.e.,

In addition to the advantages of good conversions and low acetyleneproduction at moderate temperatures, the chromic fluoride catalyst hasthe advantage of ready reactivation, which permits use for long periodswithout replace-'- ment. Reactivation (i. e., removal of deposits) ismost conveniently carried out by'heating the catalyst at temperaturesbetween about 500 and about 850 C. in a'stream of air or oxygen.

The fluoroalkenes produced by the process of this invention are usefulas intermediates in the synthesis of fluorohyd-rocarbons. Members ofthis class of organic materials may be polymerized or copolymerized toform valuable plastics or resins. A particularly valuable fluoroalkeneisvinyl fluoride, from which polymers of excellent phyiscal propertiesand unusual chemical inertness may be prepared.

As many apparently widely diflerent embodiments of this invention may bemade without departing-from the spirit and scopethereoi, it is to beunderstood that the invention is not limited to the specific embodimentsthereof except as defined in the appended claims.

We claim: v

1. Process of dehydrofluorinating a polyfluoroalkane containing 2-4carbon atoms, inclusive, with at least 2 fluorine atoms attached to 1carbon atom and having no substituents other than fluorine, whichcomprises heating said poly- 2. Process of dehydrofluorinating apolyfluoroalkane containing 2-4 carbon atoms, inclusive,

. with at least 2 fluorine atoms attached to 1 carbon atom and having nosubstituents other than fluorine, which comprises heating saidpolyfluoroalkane at a temperature of 500 C.-'l50 C. in the presence ofchromium trifluoride.

3. Process of dehydrofluorinating 1,1-difluoroethane which comprisesheating said 1,1-difluoroethane at a temperature of 400 C.-850 C. in thepresence of chromium trifluorida,

4. Process of dehydroiiuorinating 1,1-difluoroethane which comprisesheating said 1,1'-difluoro-- ethane at a temperature of 500 0,-750" C.in the presence .01 chromium trlfluoride.'

.5. Process of dehydrofluorinating 2,2-difluorobutane which comprisesheating said 2,2-di

operating temperature is aboveabout 300. However, particularly in largescale operations in: volving reactors oi considerable size, it isdesirable to use a sufliciently high linear velocity so that the natureof the flow is turbulent rather than viscous, -i. e., the Reynoldsnumber is above about 4000. Under turbulent flow'conditions thepolyfluoroalkane is .heated more uniformly and higher conversions to thedesired fluoroalkene with less formation of by-products may be obtained.

fluorobutane at a temperature of 500 C.-7 C. in the presence of chromiumtrii'iuoride.

DONALD DRAKE COFFMAN.

RICHARD D. (mama.

REFERENCES CITED 7 The following references are of record in the iile ofthis patent:

tmrnm sums PATENTS OTHERREFERENCES Tarkington at 0.1., Trans. .l 'aradaysoc. vol. 41, sac-r (1m).

